Reflective optical system

ABSTRACT

A lens arrangement is presented. The lens arrangement comprises a first element having a concave reflective surface and defining an optical axis of the lens arrangement, and a second substantially flat and at least partially reflective element spaced-apart from the first element along the optical axis. The second element is configured to allow light passage therethrough and is oriented with respect to the optical axis and the first element such that at a predetermined angle of incidence of an input light beam onto the second element, the input light beam is reflected onto the reflective surface of the first element and reflected therefrom to pass through the second element.

FIELD OF THE INVENTION

This invention relates to a reflective optical system for use inmeasuring/inspection systems.

BACKGROUND OF THE INVENTION

Various types of optical systems utilize reflective objectives. Thesesystems include for example, telescopes, UV microscopes, measurementsystems operating in UV spectral range, etc.

The commonly used reflective objective is the Schwarzschild-typeobjective. As illustrated in FIG. 1, this objective includes a firstapertured spherical mirror M₁ and a second spherical mirror M₂accommodated in a spaced-apart relationship along an axis of symmetry ofthe mirrors. The second mirror M₂ is typically mounted on a so-calledspider (shown schematically), which complicates the construction.Moreover this configuration suffers from diffractive effects that mightoccur on the spider, obscuration caused by the mirror M₂ and spiderwhich results in about 20% aperture reduction, relatively high-anglereflection which causes polarization distortion, a high minimalnumerical aperture (about 0.3) with significant angular obscuration nearthe optical axis.

Various imaging/measurement techniques require operation with a broadspectrum, i.e., from infra-red (IR) to deep ultraviolet (DUV). In thisconnection, refractive optics, due to limited availability oftransparent materials at short wavelengths, cannot be used in a spectralrange down to 190 nm. Moreover, refractive optics, due to unavoidabledispersion effects, causes high chromatic aberrations and thus does notenable illumination of a small spot needed for measuring in smallmeasurement sites.

SUMMARY OF THE INVENTION

There is a need in the art to facilitate measuring/inspecting articlesby providing a reflective optical system operable as an objective lensarrangement within a broad spectral range, from DUV or Vacuum UV (VUV)to NIR or IR.

According to one broad aspect of the invention, there is provided a lensarrangement comprising: a first element having a concave reflectivesurface and defining an optical axis of the lens arrangement, and asecond substantially flat and at least partially reflective elementspaced-apart from the first element along the optical axis, the secondelement being configured to allow light passage therethrough and beingoriented with respect to the optical axis and the first element suchthat at a predetermined angle of incidence of an input light beam ontothe second element, the input light beam is reflected onto thereflective surface of the first element and reflected therefrom to passthrough the second element.

It should be noted that the first element having a concave surface maybe configured as spherical, aspherical or parabolic type element.

According to one embodiment, the second element is an apertured mirror.This apertured mirror is accommodated such that the optical axis passesthrough the aperture, and at the predetermined angle of incidence of theinput light beam onto the reflective surface of the second element, thebeam is reflected onto the reflective surface of the first element andreflected therefrom to pass through the aperture in the second element.

Preferably, a lower surface of the apertured mirror (outer surface withrespect to the first element) and its inner surface defined by theaperture are substantially anti-reflective, more preferably aresubstantially light absorbing. Preferably, the aperture in the secondelement has a substantially conical geometry with a cone base being at alower surface of the second element.

According to another embodiment, the second element is a beam splitter.In this case, a shielding element is preferably provided configured soas to define a non-opaque region surrounded by opaque region of theshielding element. The shielding element is accommodated such that theoptical axis passes through the non-opaque region. The shielding elementmay be implemented as an opaque coating on the surface region of thebeam splitter defining the non-opaque region surrounded by the opaquecoating.

The second element is positioned with respect to the optical axis so asto be inclined to the optical axis. The inclination angle is about 70-80degrees, preferably about 79 degrees.

The reflective system of the present invention may be both in finite andinfinite system configuration. For a finite conjugate configuration thesecond element is preferably accommodated in a back focal plane of thefirst element.

According to another broad aspect of the invention, there is provided alens arrangement comprising: a first element having a concave reflectivesurface and defining an optical axis of the lens arrangement, and asecond substantially flat element spaced-apart from the first elementalong the optical axis, the second element being configured as anapertured mirror and being oriented with respect to the optical axis andthe first element such that the optical axis passes through the apertureand such that at a predetermined angle of incidence of an input lightbeam onto the mirror surface of the second element, the input light beamis reflected onto the reflective surface of the first element andreflected therefrom towards the aperture in the second element.

According to yet another broad aspect of the invention, there isprovided a lens arrangement comprising: a first element having a concavereflective surface and defining an optical axis of the lens arrangement,and a second substantially flat element spaced-apart from the firstelement along the optical axis, the second element being configured asan apertured mirror, an outer surface of the second element and itsinner surface defined by the aperture being substantiallyanti-reflective, the second element being oriented with respect to theoptical axis and the first element such that the optical axis passesthrough the aperture and such that at a predetermined angle of incidenceof an input light beam onto the mirror surface of the second element,the input light beam is reflected onto the reflective surface of thefirst element and reflected therefrom towards the aperture in the secondelement.

According to yet another broad aspect of the invention, there isprovided a lens arrangement comprising: a first element having a concavereflective surface and defining an optical axis of the lens arrangement,and a second substantially flat element spaced-apart from the firstelement along the optical axis, the second element being configured asan apertured mirror, an outer surface of the second element and itsinner surface defined by the aperture are substantially light absorbing,the second element being oriented with respect to the optical axis andthe first element such that the optical axis passes through the apertureand such that at a predetermined angle of incidence of an input lightbeam onto the mirror surface of the second element, the input light beamis reflected onto the reflective surface of the first element andreflected therefrom towards the aperture in the second element.

According to yet another aspect of the invention, there is provided anoptical system comprising an illuminator arrangement defining a firstaperture stop of the system; a first element having a concave reflectivesurface and defining an optical axis of light propagation; and a secondsubstantially flat and at least partially reflective elementspaced-apart from the first element along the optical axis, the secondelement being configured to allow light passage therethrough and beingoriented with respect to the optical axis and the first element suchthat at a predetermined angle of incidence of an input light beam ontothe second element, the input light beam is reflected onto thereflective surface of the first element and reflected therefrom to passthrough the second element, said second element and said first aperturestop defined by the illuminator arrangement being located in conjugateplanes.

The system preferably comprises an optical polarizer assembly fordirecting illuminating light towards an article under measurements andcollecting light returned from the article. Preferably, the polarizerassembly is mounted for rotation to vary an orientation of its preferredplane of polarization. The polarizer assembly comprises a polarizerelement and preferably also a compensator element. The compensatorelement is configured for shifting a beam propagation axis towards anaxis of rotation of the polarizer unit to thereby compensate for a shiftof the beam propagation axis from the axis of rotation caused by therotation of the polarizer unit. Such a compensator may be a planeparallel plate. The compensator is spaced-apart from the polarizerelement, being located upstream or downstream thereof, with respect to adirection of propagation of illuminating light towards the article undermeasurements.

The illuminator arrangement may include first and second light sources,and a light directing arrangement. The light directing arrangement isconfigured to combine first and second light portions produced by thefirst and second light sources, respectively, and direct combined lighttowards the article under measurements. The first and second lightsources may be configured to produce the first and second light portionsof different wavelength ranges, respectively.

Preferably, the illuminator arrangement includes an apertureaccommodated in the optical path of the combined light. This apertureand the second element are located in conjugate planes. The illuminatorarrangement may include reflective relay optics. The aperture of theilluminator is preferably of a double-aperture configuration having acentral blocking region.

According to yet another aspect of the invention, there is provided apolarizer unit comprising a polarizer and a compensator accommodated ina spaced-apart relationship along an axis, the compensator beingconfigured so as to, when being rotated together with the polarizerabout said axis, shifting a light beam propagation axis towards saidaxis of rotation.

According to yet another aspect of the invention, there is provided anoptical system comprising an illuminator arrangement and an objectivelens arrangement; the illuminator arrangement comprising a light sourceassembly and an aperture assembly; the objective lens arrangementcomprising a first element having a concave reflective surface anddefining an optical axis of the lens arrangement, and a second at leastpartially reflective element spaced apart from the first element alongthe optical axis, the second element being configured to allow lightpassage therethrough and being oriented with respect to the optical axisand with respect to the first element such that at a predetermined angleof incidence of an input light beam onto the reflective surface of thesecond element, the input beam is reflected onto the reflective surfaceof the first element and reflected therefrom to pass through the secondelement, said aperture of the illuminator arrangement and said secondelement being located in conjugate planes.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carriedout in practice, a preferred embodiment will now be described, by way ofnon-limiting example only, with reference to the accompanying drawings,in which:

FIG. 1 shows the commonly used Schwarzschild-type reflective objective;

FIG. 2A schematically illustrates an example of a lens arrangementaccording to the invention;

FIG. 2B more specifically illustrates the configuration of an aperturedmirror suitable to be used in the lens arrangement of the presentinvention;

FIGS. 2C and 2D schematically illustrate two more examples,respectively, of a lens arrangement according to the invention;

FIG. 3 illustrates a telecentric optical system utilizing the lensarrangement of the present invention;

FIG. 4 schematically illustrates a measurement system of the presentinvention;

FIG. 5 more illustrates the principles of a compensator element in apolarizer unit used in the system of the present invention;

FIG. 6 illustrates one example of an illuminator arrangement suitable tobe used in the system of the present invention;

FIG. 7 illustrates another example of an illuminator arrangementsuitable to be used in the system of the present invention;

FIG. 8 illustrates the measurement system including the illuminatorarrangement of the present invention;

FIG. 9 illustrates a measurement system of the present inventionutilizing yet another configuration of the illuminator arrangement;

FIG. 10 illustrates a measurement system of the present inventionutilizing yet another configuration of the illuminator arrangement;

FIG. 11 exemplifies a configuration of a polarizer unit suitable to beused in the present invention;

FIG. 12 illustrates the spectral characteristics of light sourcessuitable to be used in the present invention;

FIG. 13 schematically illustrates how the system of the presentinvention is switched between actual measurement mode and calibrationmode; and

FIG. 14 schematically illustrates the configuration of an imaging systemsuitable to be used with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows the commonly used Schwarzschold-type reflective objective,including the first apertured spherical mirror M₁ and the secondspherical mirror M₂ mounted on a spider and accommodated in aspaced-apart relationship along an axis of symmetry of the mirrors.

Referring to FIG. 2A, there is schematically illustrated an example of alens arrangement 10 according to the invention. The lens arrangement 10is configured as a reflective objective system and is suitable for usein measuring/inspecting articles, particularly patterned articles, suchas semiconductor wafers. The system 10 is configured for operating witha broadband spectrum, namely from Vacuum UV (VUV) and/or Deep UV (DUV)up to Infrared (IR), e.g. within 190-950 nm, and is also operable withpolarized light in this broad spectrum practically without a change ofthe polarization state.

The system 10 includes a concave mirror 12 (constituting a firstreflective element) which may be of spherical, aspherical or parabolictype, and a substantially flat partially reflecting element 14(constituting a second reflective element). In the present example, theelement 14 is in the form of a mirror having an aperture 14A surroundedby reflective upper surface 14B of mirror 14, e.g., is an annularmirror. For a small field of view (FOV) system, the use of a sphericaltype mirror 12 is preferable since it is simple, cheap and enables highquality surface treatment. For a large FOV application, a parabolicmirror is preferable as enabling better compensation for aberrations.

In the examples described herein, a spherical-type mirror 12 isconsidered, but it should be understood that the present invention isnot limited to this specific configuration.

The element 12 and 14 are arranged in a spaced-apart relationship alongan optical axis OA of the system. The second element 14 is positionedwith respect to the optical axis OA and to the first element 12 suchthat at a predetermined angle of incidence of an input light beam ontothe element 14, the beam is reflected therefrom to the reflectivesurface of the first element 12 and is reflected therefrom to passthrough the aperture 14A of the second element 14. In the presentexample, the mirrors 12 and 14 are accommodated such that they face eachother by their reflective surfaces, optical axis OA passes through thecenter of the aperture 14A, and mirror 14 is located between thespherical mirror 12 and article 16 under measurements. When inoperation, all light paths are essentially symmetric about the opticalaxis on the article under measurement/inspection. Thus, the system 10 isthe so-called “on-axis” objective system.

An input light (light used for illuminating the article 16), whenentering the system 10, first impinges onto the at least partiallyreflective element 14, i.e., on the reflective surface 14B of theannular mirror 14 in the present example, and is reflected therefrom tothe inner surface of the spherical mirror 12, which reflects the lightto focus it onto the article 16 via the aperture 14A. It is assumed thatthe article 16 is located accurately in the focal plane of the mirror12. The second element 14 (e.g., annular mirror) is oriented withrespect to the optical axis OA at a certain inclination angle α selectedso as to ensure desired small angle of incidence β of the input light,reflected from mirror 14, onto mirror 12. Angle α preferably ranges from70 to 80 degrees, most preferably is about 79 degrees. Angle β isessentially (90−α). Light reflected from the article is collectedthrough the aperture 14A and propagates to the spherical mirror 12,which reflects this light onto the surface 14B of the annular mirror,which due to its tilted orientation reflects the light out of the system10. The spherical mirror 12 is preferably manufactured so as to preventor at least significantly reduce scattering of light from this mirror,which is more problematic at short-wavelengths, thereby reducingdetrimental chromatic effects. Due to small angles of reflection fromthe spherical mirror 12, it practically does not affect the polarizationof light within the entire broadband spectrum.

FIG. 2B more specifically illustrates the configuration (cross-section)of mirror 14. The outer lower surface 14C and the inner surface 14D ofthe mirror defined by the aperture are preferably substantially lightabsorbing. This is implemented by providing on these surfaces a specificcoating 15 such as a black paint. As also shown in the figure, the innersurface 14D of aperture 14A has a substantially conical geometry withthe cone base being located at the bottom side of the mirror 14. Thisconfiguration provides for eliminating or at least substantiallyreducing edge effects in collected light returned (reflected) from thearticle.

It should be noted that the annular mirror 14 may be replaced by a platebeam splitter with a coating optimized for a wavelength range. This isillustrated in FIG. 2C, showing a reflective objective lens arrangementincluding a spherical mirror 12 (constituting a first reflectiveelement) and a beam splitter 114 (constituting a second reflectiveelement). In this case, there is no angular “hole” in the incident beamcaused by aperture (14A in FIG. 2A). However, the energy loss caused bythe beam splitter 114 (about 75%) might reduce a signal-to-noise ratioof the system. This energy reduction may be compensated by increasing ameasurement time. In the configuration of FIG. 2C, light propagatingthrough the beam splitter might undesirably increase parasiticscattering effects in the system. This can be avoided by using opaqueshielding. This is illustrated in FIG. 2D, showing a reflectiveobjective lens arrangement including a spherical mirror 12 and a beamsplitter 214 having an opaque coating 214′ defining an uncoated region214A (functioning as an aperture). In the present example, an opaqueshielding element is implemented as a coating on the beam splitter, butit should be understood that the same may be achieved by using aseparate element (mask) located close to the beam splitter. The opaqueshielding element blocks light impinging thereon within all the regionsoutside the aperture-region (i.e., transparent region).

Reference is made to FIG. 3 showing how the lens arrangement 10 can beused in a telecentric optical system 100. The system 100 includes anoptical lens arrangement 18 of an illuminator and the objective lensarrangement 10. The illuminating lens arrangement 18 includes a relaylens 18A (preferably a spherical mirror), and an aperture 18B locatednear a focal plane of the relay lens (spherical mirror) 18A. Theaperture 18B of the illuminator is located at a position such that therelay lens (spherical mirror) 18A preferably images the aperture 18B tothe back focal plane of the objective lens (spherical mirror) 12, i.e.,adjacent to the mirror 14. It should be noted that the lens arrangementof the present invention can be used both with finite conjugate imagingand infinite conjugate imaging that include a tube lens as well.Additionally, folding mirrors 19A and 19B or other beam directingassemblies may be used in order to provide a desired configuration(size, light propagation direction, etc.) of the optical system 100.

FIG. 4 schematically illustrates an optical measurement system 200utilizing the reflective objective system 10 of the present invention.The system 200 includes an illuminator arrangement 20; aspectrometer-type detection unit 22; and a light directing unit 24configured for focusing incident light onto an article undermeasurement/inspection and collecting light from the article to thedetection unit. The system 200 preferably also includes an imagingsystem 26. Further provided in the measurement system 200 is a controlsystem 28 connectable to various elements of system 200 for controllingtheir operation and processing measured data.

The illuminator arrangement 20 is configured for producing broadbandillumination, which for the purposes of the present invention(inspection/measurements in patterned articles) is of about 190-950 nm.Various configurations of the illuminator arrangement will be describedfurther below.

The light directing unit 24 includes a beam splitter/combiner 23A; anoptical polarizer unit 23B; and a reflective objective arrangement 10.Also preferably provided in the unit 24 is an aperture 23C serving as afield stop. Considering the use of an imaging system, the lightdirecting unit 24 also includes a beam separator such as a pinholemirror 23D accommodated in the optical path of light returned from thearticle in order to transmit a part of light impinging on the pinhole tothe spectrometer 22 and reflect the other part of this light to theimaging system 26. It should be understood that such a beam separationmay also be implemented by replacing the pinhole mirror by a standardbeam splitter. An example of the imaging system 26 will be describedfurther below with reference to FIG. 14.

The optical polarizer unit 23B includes a polarizer element 29 and acompensator element 30. The polarizer unit 23B is operated by thecontrol unit 28 so as to provide a desired polarization of light.

As shown more specifically in FIG. 5, the provision of the compensator30 is associated with the following. In order to prevent undesiredeffects (ghost) caused by reflections from the facets of the polarizer,the polarizer is tilted in relation to the optical axis OA of the systemat a small angle θ^((p)) _(tilt) (e.g., of about 2.5°). Due tomanufacturing tolerances, polarizer's facets PF₁ and PF₂ are notsubstantially parallel, forming a small optical wedge (about 1′) whichis schematically shown as an angle W_(p). Rotation of the polarizerpractically causes a shift of the light beam L′ from the axis ofrotation AR of the polarizer (which is substantially coinciding with theoptical axis OA of the system), which will result in a shift d of theilluminated spot on the article (e.g., wafer). In order to compensatefor such a shifting, the compensator 30 configured like an optical wedgeis used, where the outer facet CF₁ of the compensator 30 forms an angleW_(c) (of about 1′) with respect to the opposite facet CF₂, and thecompensator 30 is tilted a small angle θ^((c)) _(tilt) (e.g., of about8°) in relation to the optical axis OA. In that case, a light beampropagates along a “folded” path L_(comp) and an illuminated spot on thearticle is not shifted relative to the axis of polarizer rotation AR.During assembling the polarizer unit 23B, the compensator element 30 andthe polarizer element 29 are rotated with respect to each other in orderto compensate the effects of polarizer tilt tolerances, and/or polarizeroptical wedge, and/or polarizer length tolerances, by providing theappropriate orientation of the polarizer and compensator wedges.

Reference is made to FIG. 6 exemplifying the configuration of theilluminator arrangement 20. In the present example, the illuminatorarrangement 20 includes a light source assembly, generally at 32, and alight directing assembly 34. The light source assembly 32 includes afirst light source 32A capable of producing a first light portionincluding a visual spectral range, e.g., Xe-lamp or QTH lamp; and asecond light source 32B capable of producing a second light portionincluding DUV (VUV) spectrum, e.g., Deuterium lamp. The first and secondlight sources are preferably selected such that the spectral ranges ofthe first and second light portions partially overlap so as to provide adesired intensity distribution over the required spectral range. Anexample of the spectral characteristics of light sources 32A and 32Bwill be described further below with reference to FIG. 12.

The light directing assembly 34 includes a reflective relay lensarrangement 36 which in the present example is constituted by aspherical mirror; an aperture 38 accommodated near the back focal planeof the relay lens arrangement 36; and includes optional folding mirrors40A and 40B. The relay lens arrangement 36 directs light coming from thelight source assembly (first and second light portions) to the firstfolding mirror 40A, which reflects the light to the second foldingmirror 40B, which in turn directs the light towards the light directingunit (24 in FIG. 4), namely towards the beam splitter/combiner 23A. Itshould be understood that the provision of folding mirrors is optional.In the example of FIG. 6, the aperture 23C is located in the opticalpath of light propagating from mirror 40A to mirror 40B, but it shouldbe noted that the aperture 23C may alternatively be located downstreamof mirror 40B, namely at the output of the illuminator arrangement 20 asshown in FIG. 4.

FIG. 7 shows another example of combining illumination from two lightsources 32A and 32B. As shown, light from lamp 32A is directed via anoptical fiber 44 to a refractive relay lens arrangement 46, whichdirects this light towards the lamp 32B via folding mirrors 48A and 48B.Turning back to FIG. 6, it should be understood that using the lightdirecting scheme of FIG. 7 in the illuminator arrangement 20, theelement denoted 32A located proximate to lamp 32B upstream thereof wouldbe constituted by the folding mirror 48B.

The aperture 38 of the illuminator arrangement (of either one of theexamples herein described) is conjugate with the back focal plane of thereflective objective 10 and preferably also with the apertured secondelement (14 in FIG. 2).

For the preferred embodiment of the invention, where the reflectiveobjective system utilizes an annular mirror 14, the aperture 38 ispreferably formed with a central blocking region surrounded by theaperture gap, i.e., is the so-called double-aperture assembly. This isillustrated in FIG. 8, exemplifying the entire measurement system of thepresent invention. As shown, the aperture 38 has a central blockingregion 38A, thus preventing illuminating light from being incident ontothe opening 14A in the mirror 14, and allowing only the reflection ofthe illuminated light from the spherical mirror 12 to propagate towardsthe aperture 14A.

Referring to FIG. 9, there is illustrated another example of ameasurement system 300. To facilitate understanding, the same referencenumbers are used in all the figures for identifying those componentswhich are common in all the examples of the invention. The system 300includes an illuminator arrangement 320; a spectrometer-type detectionunit 22; a light directing unit 24; and preferably also includes animaging system 26, in which case a beam separating element 23D (e.g.,pinhole mirror) is appropriately provided to define imaging andmeasurement channels. In this example, the illuminator arrangement 320includes two light sources 32A and 32B operating in different wavelengthranges as described above; and a light directing assembly 34 including arefractive relay lens 46A and folding mirror 49A directing the firstlight portion from lamp 32A to lamp 32B, another relay lens 46B andfolding mirror 49B to direct the combined light from both sources, andan aperture assembly 50 with variable apertures of various diameters andshapes. As shown in the figure, system 300, similarly to system 200, ispreferably equipped with a field stop aperture 23C.

FIG. 10 exemplifies yet another configuration of a measurement system400. System 400 is generally similar to the previous examples, butdistinguishes therefrom in the configuration of its illuminatorarrangement 420. The latter includes different light sources 32A and 32Band a beam combiner 52 for combining the first and second light portionsemitted by light sources 32A and 32B, respectively; and a relay lens 46.

Reference is now made to FIG. 11 schematically illustrating a preferredconfiguration of the polarizer 29 suitable to be used in the measurementsystem of the present invention. This polarizer is the subject of aco-pending application IL 160724, assigned to the assignee of thepresent application. The polarizer 29 is formed by two prisms P₁ and P₂attached to each other by an optical glue layer GL between the tiltedsurfaces of S₁ and S₂ of the prisms P₁ and P₂, respectively. In thisdevice, the following parameters are appropriately selected to ensurethe device operation for light within the spectral range of about 190nm-950 nm: cut angle θ′ of the prism; and the properties of the gluematerial layer GL.

The prisms P₁ and P₂ are made of a birefringent material that istransparent for the required broadband spectral range, and is preferablyα-BBO or quartz. The prisms are configured such that the preferred axisPA of the prism material forms a predetermined angle θ′ (cut angle) withthe tilted surface S₁ of the prism P₁ by which it is coupled to theother prism P₂. The glue material for the layer GL located betweensurfaces S₁ and S₂ is selected so as to be characterized by a dispersionprofile n_(g)(λ) matching the dispersion profiles n_(e)(λ) and n_(o)(λ)of the prism material for, respectively, extraordinary and ordinary raysR_(o) and R_(e) in the required spectral range. Moreover, the gluematerial is selected to be stable over time when exposed to variationsin environmental conditions (temperature variations, UV radiation,etc.). For α-BBO crystal prisms, the preferred glue material is aSilicon RTV transparent to electromagnetic radiation ranging from 190 nmto 950 nm. Such glue may be CV15-2500, commercially available from NuSilTechnology, USA. A 50 μm layer of this glue has about 95% transparencyover the whole DUV to NIR spectrum.

FIG. 12 exemplifies the spectral characteristics (intensity of emittedlight as a function of wavelength) of light sources 32A and 32B. Threegraphs G₁, G₂, G₃ are shown corresponding to the spectralcharacteristics of, respectively, Xe-lamp, Deuterium lamp, and thecombined light from both lamps (Xe:De ratio is 1:20). Thus, the combinedlight includes the spectrum of two different lamps with sufficientintensity within the entire spectral range.

FIG. 13 illustrates the main principles of another aspect of theinvention consisting of monitoring the intensity of light produced bythe entire light source assembly (two-lamp assembly). As shown, twocontrollable shutters 56 and 58 and a mirror 60 are provided so as toselectively allow and prevent the illuminating light propagation towardsthe article under measurement/imaging. In the actual measurement mode,shutter 56 is in its operable (opened) position and shutter 58 isinoperative (closed) as shown in the figure in dashed lines. Hence, theilluminating light propagates towards the article. In the monitoringmode, as shown in the figure, shutter 58 is operative (open) and shutter56 is inoperative (closed), in which case the illuminating light isprevented from reaching the article while the portion of theilluminating light that is reflected by the beam splitter 23A isreflected by mirror 60 towards the pinhole 23D, i.e., towardsmeasurement and imaging channels. This enables to monitor and calibratethe illuminating light parameters and thereby improve the measurementstability. It should be understood that shifting the shutter between itsoperative and inoperative positions may be achieved by any suitablemechanism, e.g., by mounting the shutter for rotation with respect tothe optical path of light (as shown in the figure). The same controlsystem (28 in FIG. 4) may be used for controlling the shutter operation.

FIG. 14 exemplifies the configuration of the imaging system 26 suitableto be used in the present invention. Generally, the imaging system isconfigured to acquire images of the article and to implementautofocusing. The imaging system 26 includes a light source assembly 70,detection unit 72 and a light directing system 74. The latter includes acondenser lens assembly 76 (two-lens assembly in the present example); agrid assembly 78 (for autofocusing); collimating/focusing lenses 80A and80B, a beam splitter/combiner 82, and an imaging lens 84. Light from thelight source 70 is directed by lenses and beam splitter to the pinholemirror 23D which reflects this light towards the article. Light comingfrom the article is reflected by the pinhole mirror and then by the beamsplitter 82 towards the detection unit 72. The use of a grid assemblyfor the autofocusing is described in U.S. Pat. No. 5,604,344 assigned tothe assignee of the present application, and is incorporated herein byreference with respect to this specific example only. The operation ofthe imaging system does not form part of the present invention, andtherefore need not be more specifically described.

Thus, the present invention provides for measuring/inspecting articleswith a broad illuminating spectrum, i.e., from IR to DUV (or VUV), usingessentially reflective optics. This is due to the novel configuration ofthe objective lens arrangement. This configuration also allows forimplementing a telecentric optical system. The invention allows formeasurement/inspection/imaging with polarized light, which is improveddue to the novel configuration of a non-polarizing reflective objectivelens arrangement and a polarization assembly using a polarizer and acompensator.

The present invention provides for spectrometry, spectrophotometry,reflectometry and ellipsometry measurement/inspection of articles, whichis especially useful for patterned articles, for measuring theparameters of the pattern (e.g., thickness of layers). The inventionalso provides for scatterometric measurements of periodic patternedstructures. It should be understood that the technique of the presentinvention may be in various applications, for example formeasuring/inspecting semiconductor wafers, reticles, flat paneldisplays, etc. The system of the present invention has a compactconfiguration and can therefore be advantageously used as an integratedmeasurement system, for example being associated with a lithographytools arrangement, Chemical Mechanical Planarization (CMP) system,material deposition system (CVD, PVD), material removal system (etchingtools arrangement), etc.

Those skilled in the art will readily appreciate that variousmodifications and changes can be applied to the embodiments of theinvention as hereinbefore described without departing from its scopedefined in and by the appended claims.

1. An optical system for use in measurement/inspection of an article,the optical system comprising an illuminator arrangement, and a lightdirecting unit configured for focusing incident light beam from saidilluminator arrangement onto an article under measurement/inspection atsubstantially normal incidence and for collecting light from thearticle, said light directing unit comprising an objective arrangementcomprising a first reflective element having a concave reflectivesurface and defining an optical axis of the incident light propagationtowards the article, and a second substantially flat and at leastpartially reflective element spaced-apart from said first reflectiveelement along said optical axis, the second element being configured toallow light passage therethrough and being oriented with respect to theoptical axis and the first element such that at a predetermined angle ofincidence of the incident light beam from said illuminator arrangementonto the second element, said incident light beam is reflected onto thereflective surface of the first element and further reflected therefromto pass through the second element and focus on the article.
 2. Thesystem of claim 1, wherein the concave reflective surface of said firstreflective element is symmetric about the optical axis defined thereby.3. The system of claim 2, wherein said first reflective element is aspherical mirror.
 4. The system of claim 2, wherein said firstreflective element is an aspheric mirror.
 5. The system of claim 1,wherein said illuminator arrangement comprises at least one light sourceand light directing optics with an aperture stop, said aperture stop andthe second element of the objective arrangement being located inconjugate planes.
 6. The system of claim 1, wherein said illuminatorarrangement comprises at least one light source and light directingoptics, said light source being located in a plane conjugate with aplane of location of the article under measurement/inspection.
 7. Thesystem of claim 1, wherein said illuminator arrangement comprises atleast one light source and light directing optics with a field stop,said field stop being located in a plane conjugate with a plane oflocation of the article under measurement/inspection
 8. The system ofclaim 7, wherein the light source and the field stop are located inconjugate planes.
 9. The system of claim 1, wherein the second elementis an apertured mirror oriented such that the optical axis passesthrough the aperture in said mirror, and at the predetermined angle ofincidence of the input light beam onto the reflective surface of thesecond element, the input light beam is reflected onto the reflectivesurface of the first element and reflected therefrom towards theaperture in the second element.
 10. The system of claim 9, wherein anouter surface of the second element and its inner surface defined by theaperture are substantially light absorbing.
 11. The system of claim 1,wherein the second element is accommodated in a back focal plane of thefirst element.
 12. The system of claim 1, comprising an opticalpolarizer unit for affecting state of polarization of said incidentlight beam and light collected from the article.
 13. The system of claim12, wherein the polarizer unit is mounted for rotation to vary anorientation of its preferred plane of polarization.
 14. The system ofclaim 13, wherein the polarizer unit comprises a polarizer element and acompensator element, the compensator element being configured forshifting a propagation axis of a light beam impinging thereon towards anaxis of rotation of the polarizer unit to thereby compensate for a shiftof the beam propagation axis from the axis of rotation caused by therotation of the polarizer unit.
 15. The system of claim 14, wherein thecompensator is configured substantially as a plane parallel plate and istilted in relation to the optical axis to shift the beam propagationaxis.
 16. The system of claim 1, wherein the illuminator arrangementcomprises first and second light sources and a light directingarrangement, the light directing arrangement being configured to combinefirst and second light portions produced by the first and second lightsources, respectively, and direct combined light towards the articleunder measurements.
 17. The system of claim 16, wherein the first andsecond light sources are configured to produce the first and secondlight portions of different overlapping wavelength ranges, respectively.18. The system of claim 17, wherein the illuminator arrangementcomprises all-reflective relay optics.
 19. The system of claim 5,wherein the aperture of the illuminator has a double-apertureconfiguration with a central blocking region.
 20. An on-axis opticalsystem for use in measurement/inspection of an substantially planararticle such as a semiconductor wafer, reticle or flat panel display,the on-axis optical system comprising an illuminator arrangement with alight source assembly and an aperture assembly, a light directing unitconfigured for focusing incident light beam from said illuminatorarrangement onto said article under measurement/inspection atsubstantially normal incidence and for collecting light from thearticle, said light directing unit comprising an objective arrangementcomprising a first element having a concave reflective surface anddefining an optical axis of incident light propagation, and a secondsubstantially flat and at least partially reflective elementspaced-apart from the first element along the optical axis, the secondelement being configured to allow light passage therethrough and beingoriented with respect to the optical axis and the first element suchthat at a predetermined angle of incidence of the incident light beamfrom said illuminator arrangement onto the second element, said lightbeam is reflected onto the reflective surface of the first element andfurther reflected therefrom to pass through the second element and focuson the article, said aperture of the illuminator arrangement and saidsecond element being located in conjugate planes.